FIG. 1 schematically illustrates a rear wheel drive vehicle powertrain with an independent rear suspension. Solid lines indicate shafts capable of transferring torque and power. Engine 10 converts chemical energy in the fuel into mechanical power. Transmission 12 modifies the speed and torque to suit current vehicle requirements. At low vehicle speed, the transmission provides torque multiplication for improved performance. At cruising vehicle speed, the transmission increases speed permitting the engine to run at a fuel efficient operating point. The output of transmission 12 is coupled to the input of differential 14 by rear driveshaft 16. Two components are coupled when rotating either component by one revolution causes the other component to rotate by one revolution. Differential 14 distributes the power to left rear wheel 18 and right rear wheel 20 via left axle shaft 22 and right axle shaft 24 respectively. Differential 14 changes the direction of rotation by 90 degrees and multiplies the torque by a final drive ratio. Differential 14 provides approximately equal torque to each wheel while permitting slight speed differences as the vehicle turns a corner.
In a four wheel drive vehicle based on the powertrain of FIG. 1, a transfer case fixed to the transmission divides power between the rear driveshaft 16 and a front driveshaft that directs power to the front wheels via a front differential. In a front wheel drive powertrain, the front differential is typically integrated with the transmission in an assembly called a transaxle. In a four wheel drive vehicle based on a front wheel drive powertrain, a power take-off unit fixed to the transaxle drives a rear driveshaft and a rear drive unit fixed to the rear differential selectively transfers power to the rear differential. Throughout this document, the term transmission should be interpreted to include any transfer case or power take-off unit. Similarly, the term differential should be interpreted to include any rear drive unit.
Engine 10, transmission 12, and rear differential 14 are mounted to vehicle structure. Wheels 18 and 20 are supported via a suspension that allows the wheels to move vertically over road bumps while limiting the vertical movement of the vehicle body. The axis of rotation of engine 10 and transmission 12 may be offset slightly from the input axis of differential 14. Universal joints 26 and 28 accommodate this offset by transmitting torque and power between shafts that rotate about intersecting but not coincident axes. Similarly, universal joints 30, 32, 34, and 36 accommodate the offset between the output axis of differential 14 and the axes of rotation of wheels 18 and 20 even though the axes of rotation of the wheels fluctuates as the wheels absorb road bumps. In some rear wheel drive vehicles, the differential 14 is not mounted directly to the vehicle frame but is instead supported by left and right axles 22 and 24. This eliminates the need for universal joints 30 and 34 but universal joints 26 and 28 must then accommodate a fluctuating offset between the transmission output axis and the differential input axis.
A variety of types of universal joints are known. In the simplest types of universal joint, although the driving shaft and driven shaft are coupled, the instantaneous speed of the driven shaft differs slightly from the instantaneous speed of the driving shaft as a function of rotational position. Consequently, although the driving shaft may have a constant speed, the driven shaft speed may oscillate at a frequency proportional to the driving shaft speed. Due to the inertia associated with the driven shaft, this results in an oscillating torque level. The oscillating torque level may be perceived by vehicle occupants, especially if the frequency is close to a natural frequency of the driveline. Therefore, universal joints that maintain equal instantaneous speeds between the driving and driven shafts, called Constant Velocity (CV) joints, are desirable. Several types of CV joint mechanisms are known. Among known CV joint types, tripod joints and Rzeppa joints are common in automotive drivelines.